As others have mentioned, there's often times a catch or fine print with hosting plans that offer unlimited storage. If you're considering a hosting plan with unlimited storage (and truly need lots of storage), you should check their Terms Of Service and Fair Usage Policy for certain restrictions, such as limits regarding I/O usage, IOPS, processes, CPU usage, etc.
The hosting companies offering unlimited disk will usually have a lot of conditions attached to it. In most cases, their condition is that the space can be used only for the website. For example, you cannot use the space as a sort of offsite backup space for your home or office computer.